In my U.S. Pat. No. 5,241,328, a digital printing system is described which uses the heat energy of a laser beam to print a finely detailed image onto a receiver element, which, for example, is a slide transparency. The apparatus and method described therein provides improved linearity in tone scale compared to previous images of that kind. The printed image, which should be as faithful as possible to an original or master image, is printed as a series of dots or pixels by scanning the laser beam a line at a time across a dye donor element held by vacuum against a blank slide transparency or receiver element. The laser beam is rapidly modulated in intensity by input data corresponding to the master image and "spots" or pixels of dye, corresponding to the pixels of the master image, are transferred by thermal energy from the dye donor element onto the slide transparency or receiver. To obtain sufficiently high resolution, such as needed in photographic prints or slide transparencies, to give a desired degree of sharpness in the image (e.g., at 100 power magnification for slides), the dye pixels are very small, e.g., only about 7 microns, and are written at a very close pitch, for example, about 4,000 pixels per inch. They are written at a high speed (e.g., 140 KHZ) since otherwise the printing of the entire image would take an inordinately long time.
As disclosed in the above-referenced application, for a thermally printed reproduction of a master image to be suitable for viewing at projection magnifications of up to 100X, it is necessary that the individual pixels of the printed image be very small and very accurately positioned. It is also highly desirable that the individual pixels each have a density or tone value corresponding as closely as possible to the respective densities of the pixels of the master image. It is also necessary, as noted above, that the speed of producing the image be maximized to reduce the time necessary for the production of each image.
As also disclosed in the above-identified application, the apparatus provided comprises a laser and an electronic laser drive control and logic circuit (LDCL) for thermally printing from one or more dye donor elements onto a receiver element such as a slide transparency or photographic print. The electronic LDCL circuit is uniquely configured to be able to drive the laser over a wide range of frequencies which print data "words" whose weighted digital values correspond to the pixels of a master image. The LDCL circuit, in executing each "word" for a pixel immediately drives a laser from a threshold near-on value to an optimum "full-on" value, and then leaves the laser full-on for a time corresponding to the weighted digital value of that respective binary word. By way of example, for a word of eight bits there are 256 different levels of values. A tone scale divided into such a number of discreet values appears visually to be substantially continuous. The full-on power level of the laser is set at an optimum operating point above the level at which dye is transferred from a donor element and prints as dye pixels on the receiver element. There is a substantially linear relation between the density of each printed dye pixel on the receiver element and the weighted digital value of the respective data word which drives the laser control circuit. Moreover, the substantially linear relation is still present even though there otherwise may be minor variations in the thermal characteristics of the dye over the surface area of a donor element, or from one donor element to another, or minor variations from one laser to another. In that system, the laser drive and control logic circuit (LDCL) includes means for turning laser full-on to an optimum power level for a time determined by high speed picture data in the form of the binary words. An asynchronous load counter in the LDCL is loaded with each picture data binary word and then, under control of a high speed clock, "counts" during an interval of time the weighted value of the binary word. This count interval determines the time-on of the laser. A count divider in the LDCL determines the loading of each binary word into the counter and turns on a drive unit which powers the laser. The drive unit is turned off by the counter at the end of the count interval. This method of controlling the operation of the laser is called pulse width modulation (PWM). While PWM has been found to provide the advantages noted in the referenced co-pending application, it has been found that the productivity of a system using PWM can be significantly improved according to the present invention. This results from the fact that many images, or portions of images, to be generated by printers utilizing PWM do not require that any of the pixels be exposed for the maximum time possible, but that a maximum exposure can be determined for a selected portion of the image, whether it is for a line, for a paragraph, or for a selected color, that can then be used to speed up the printing of that portion of the image.